1. Field of the Invention
The present invention relates to a semiconductor device including a delay circuit.
2. Description of the Prior Art
In the semiconductor device, timing adjustment of signals for controlling circuit operation is usually performed by using the delay circuit.
For example, as shown in FIG. 24, a conventional delay circuit includes an input portion 3 for receiving a signal, a first delay portion 6 formed by two inverters 5a, a first output portion 4a for outputting a first delay signal produced by the first delay portion 6, a second delay portion 7 connected to the input portion 3 in parallel with the first delay portion 6 and formed by four inverters 5a, a second output portion 4b for outputting a second output signal produced by the second delay portion 7, a first metallic line ME1 for connecting the first output portion 4a and an output portion 4 for outputting a delay signal of the delay circuit and a second metallic line ME2 for connecting the second output portion 4b and the output portion 4. Only one of the first and second metallic lines ME1 and ME2 is used actually through its selection during production of the semiconductor device. In this conventional delay circuit, since the number of the inverters 5a used for the second delay portion 7 is larger than that of the first delay portion 6, delay amount of the signal on the second metallic line ME2 is larger than that of the first metallic line ME1.
Conventionally, since delay amount of the delay circuit is switched by the metallic lines or the like, it has been impossible to change the delay amount after production of the semiconductor device unless a fuse or the like is formed.
On the other hand, due to recent trend towards low-voltage operation, there is a keen demand for a semiconductor device which is usually operated at, for example, 3V but can be operated also at 2V. However, low voltage operation is prevented by such a cause as dependence of the delay circuit on voltage, thereby resulting in improper operational timing. Therefore, if delay amount is adjusted by the metallic lines as described above, it is difficult to operate the semiconductor device at voltages ranging from ordinary voltage to low voltage.
Accordingly, an essential object of the present invention is to provide, with a view to eliminating the above mentioned drawbacks of prior art semiconductor devices, a semiconductor device which is capable of generating, even if power source voltage changes, a delay signal corresponding to the power source voltage.
In order to accomplish this object of the present invention, a semiconductor device according to the present invention comprises: a power source voltage detecting means for outputting a voltage detection signal in accordance with a power source voltage; and a delay means which includes an input portion for receiving an input signal, a plurality of delay portions connected to the input portion, a selective portion for selecting one of the delay portions in accordance with the voltage detection signal and an output portion for outputting a delay signal obtained by delaying the input signal by the one of the delay portions.